Vehicles may be equipped with various exhaust after-treatment devices to reduce the release of exhaust emissions into the atmosphere. For example, three-way catalysts may reduce levels of various emissions including carbon monoxide and unburnt hydrocarbons while selective catalyst reduction (SCR) devices may be used to reduce levels of NOx emissions. To ensure the after-treatment devices are functioning optimally and emissions control standards are maintained, diagnostics of the SCR device may be periodically or opportunistically carried out. Feedback from one or more exhaust system sensors may be used to determine if a SCR device is missing due to manufacturing defects or from decoupling during vehicle operations.
Various approaches are provided for carrying out diagnostics of a SCR device. In one example, as shown in U.S. Pat. No. 8,186,146, Jayachandran et al. shows measuring exhaust temperature upstream and downstream of an exhaust after-treatment device via exhaust temperature sensors. A rate of change of temperature upstream of the exhaust after-treatment device may be compared to a rate of change of temperature downstream of the exhaust after-treatment device. Due to the thermal mass of the exhaust after-treatment device, the rate of change of temperature downstream of the exhaust after-treatment device may be substantially lower than that upstream of the device. Therefore, degradation of the exhaust after-treatment device may be indicated if the upstream rate of temperature change and the downstream rate of temperature change differ by less than a predetermined amount.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, during certain engine operating conditions, operation of an exhaust after-treatment device such as the SCR device may cause a temperature exotherm thereby increasing exhaust temperature downstream of the SCR device. In the approach shown by Jayachandran et al., during conditions when there is an exothermic reaction in the SCR device, there may be erroneous indications of absence of the SCR device due the temperature downstream of the catalyst being higher than temperature upstream of the catalyst. In another approach, for SCR device diagnostics, urea may be injected upstream of the SCR device and NOx levels upstream of the SCR device may be compared to the NOx levels downstream of the SCR catalyst. However, to provide a reliable estimate of SCR device status, a significant number of readings may need to be taken over a long duration of engine operation. During low NOx loading of the SCR device, injection of urea may negatively affect emissions quality when performed over the required longer duration.
In one example, the issues described above may be addressed by a method comprising: indicating absence of an exhaust catalyst responsive to a sensed temperature profile downstream of the exhaust catalyst different from an expected temperature profile, the expected temperature profile based on water adsorption and a related exothermic temperature increase by the exhaust catalyst. In this way, by comparing exhaust temperature downstream of the exhaust catalyst to an expected temperature profile during a cold-start condition, a missing catalyst may be reliably indicated by leveraging differences in catalyst water adsorption.
As one example, exhaust after-treatment devices such as a SCR device may be a zeolite based catalyst. During cold-start conditions, water from the exhaust may be adsorbed by the zeolite layer of the SCR device. Water adsorption by zeolite is an exothermal process causing release of heat. Due to the exothermic process of water adsorption at the SCR device, exhaust temperature sensed downstream of the SCR device may be significantly higher than the exhaust temperature sensed upstream of the catalyst. By leveraging this attribute, immediately after an engine cold-start, the presence of a SCR device may be confirmed when the exhaust temperature downstream of the SCR device is higher than the exhaust temperature upstream of the SCR device. If the SCR device is missing, the zeolite may not be able to adsorb water from exhaust and the temperature downstream of the SCR device may be substantially equal to or lower than the exhaust temperature upstream of the SCR device. Further, after engine warm-up, when water from the exhaust has been removed due to evaporation or when the zeolite in the SCR device is saturated with water, the exhaust temperature downstream of the SCR device may become substantially equal to or lower than the exhaust temperature upstream of the SCR device. Once it is confirmed that the SCR device is missing, a diagnostic code may be set and further, one or more engine operating parameters such as fueling schedule, boost pressure, torque output, etc., may be adjusted in subsequent engine cycles.
In this way, by opportunistically comparing exhaust temperature sensed upstream of an exhaust catalyst such as a SCR device to exhaust temperature sensed downstream of the SCR device, absence of the SCR device may be indicated. The technical effect of using the exothermic process of water adsorption by a zeolite layer present in the SCR device during a cold-start condition is that an existing zeolite in the SCR device may be successfully utilized for on-board detection of a missing SCR device. By adjusting engine operating parameters in response to the detection of a missing catalyst, compliance to emissions standards may be maintained until the SCR device is reinstated. Overall, by detecting a missing exhaust after-treatment device and then adjusting engine operating conditions, emissions quality, and fuel efficiency may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.